Monthly Microscopiol"] 
Journal, Aug. 1, 1869. J 
of Vitreous S]oon(/es. 
109 
mud to the order Porifera Yitrea, which I have defined in the 
'Annals and Magazine of Natural History' for February, 1860. 
This order is mainly characterized by the great variety and 
complexity of form of the spicules, which may apparently, with 
scarcely an exception, be referred to the hexradiate stellate type, a 
form of spicule which does not appear to occur in any other order 
of sponges. The genus Holtenia is nearly allied to Hyalonema, 
and seems to resemble it in its mode of occurrence. Both genera 
live imbedded in the soft upper layer of the chalk-mud in which 
they are supported, — Holtenia by a delicate range of siliceous fibres, 
which spread round it in all directions, increasing its surface 
without materially increasing its weight ; Hyalonema by a more 
consistent coil of spicules, which penetrates the mud vertically and 
anchors itself in a firmer layer. 
It appears to me and to Dr. Carpenter, who have had our 
attention specially directed to this point as bearing upon the con- 
tinuity and identity of some portions of the present calcareous 
deposits of the Atlantic with the cretaceous formation, that the 
vitreous sponges are more nearly allied to the Ventriculites of the 
chalk than to any recent order of Porifera. We are inchned to 
ascribe the absence of silica in many ventriculites, and the absence 
of disseminated silica in the chalk generally, to some process, pro- 
bably dialytic, subsequent to the deposit of the chalk, by which 
the silica has been removed and aggregated in amorphous masses, 
the chalk flints. 
The vitreous sponges along with the living Khizopods and other 
Protozoa which enter largely into the com]30sition of the upper 
layer of the chalk-mud, appear to be nourished by the absorption 
through the external surface of their bodies of the assimilable 
organic matter which exists in appreciable quantity in all sea- water, 
and which is derived from the life and death of marine animals 
and plants, and, in large quantity, from the water of tropical rivers. 
One principal function of this vast sheet of the lowest type of 
animal life, which probably extends over the whole of the warmer 
regions of the sea, may probably be to diminish the loss of organic 
matter by gradual decomposition, and to aid in maintaining in the 
ocean the " balance of organic nature." — Abstract of a 'pa]per read 
before the Boyal Society at its last meeting, June 23rd. 
